[gcc r15-7758] libstdc++: Improve optional's <=> constraint recursion workaround [PR104606]

[Patrick Palka]

https://gcc.gnu.org/g:815f1f27a1dba2f0acd1f02d0beafedadebe967c

commit r15-7758-g815f1f27a1dba2f0acd1f02d0beafedadebe967c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Feb 28 10:15:45 2025 -0500

    libstdc++: Improve optional's <=> constraint recursion workaround [PR104606]
    
    It turns out the reason the behavior of this testcase changed after CWG
    2369 is because validity of the substituted return type is now checked
    later, after constraints.  So a more reliable workaround for this issue
    is to add a constraint to check the validity of the return type earlier,
    matching the pre-CWG 2369 semantics.
    
            PR libstdc++/104606
    
    libstdc++-v3/ChangeLog:
    
            * include/std/optional (operator<=>): Revert r14-9771 change.
            Add constraint checking the validity of the return type
            compare_three_way_result_t before the three_way_comparable_with
            constraint.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 libstdc++-v3/include/std/optional | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index 832dc6fd84ba..a616dc07b107 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1685,7 +1685,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _Tp, typename _Up>
     requires (!__is_derived_from_optional<_Up>)
-      && three_way_comparable_with<_Up, _Tp>
+      && requires { typename compare_three_way_result_t<_Tp, _Up>; }
+      && three_way_comparable_with<_Tp, _Up>
     constexpr compare_three_way_result_t<_Tp, _Up>
     operator<=> [[nodiscard]] (const optional<_Tp>& __x, const _Up& __v)
     { return bool(__x) ? *__x <=> __v : strong_ordering::less; }